Well, you seem to know more about the tech than I do, but I have a JamMan Stereo that can do it that's many years old, as can my Trio+ (couple years old as well). Again, maybe it is too processor-intensive, but I still have my fingers crossed - because, again, what use is stretching the time if it also makes everything you play on top of it out of tune by changing pitch?
It’s certainly not new tech, Ableton has been doing it since day one, though surprisingly a lot of software is only now getting around to it, like my NI Maschine finally started to have real time audio time stretch, so you can adjust audio loops to tempo.
I think it’s essentially just chopping the audio up into little slices and adding gaps of time, keeping the pitch, transients, etc. The “complicated” part is where to add gaps of time since you can’t just evenly space it or you’d lose the rhythm. Ableton etc looks at the loop, defects the beats, and adds markers which it uses to determine the bpm, then can use that into to sync to another bmp.
That’s how you can do it well, with a very natural sound quality, doesn’t seem processed.
Other software, like a lot of dj software can do tempo adjustment while trying to keep the key of the track sounding natural, but all it does is kind of a crude pitch shift effect. Play the track faster to match bmp, vocals start to sound like chipmunks, so it can pitch it down a bit to sound more natural, but then it just sounds synthetic, it’s the right bmp and pitch, but often not very pleasant sounding, imo
I think the Axe could adjust looper speed and then run a pitch block to account for the change in loop speed, but it’s not going to sound quite “right” to the picky ears of most guitarist.
If you can take the loop, analyze it, chop up the audio, and then play those slices back in real-time with a different tempo it would work, but I don’t know if the Axe software really is able to handle that type of algorithm. Some stuff that’s easy to do in a PC environment with a DAW can be tricky to do in a dedicated DSP environment.
Again, I’m not saying it’s not possible, but I think it would take a lot of processing resources and probably a good bit of coding, to make it work well and sound natural.
I think it would be sweet too to have a bank of loops stored in the unit and you could recall those on the fly, have them match the tempo your playing at etc, but the Axe certainly can get an added storage ability, short of the USB port being able to read a flash drive, which again while easy in a PC environment may not have the framework needed for usb mass storage.
“Best” solution I’ve found is simply using Ableton via a laptop. Then you can do looping, time stretch, trigger clips on the fly etc. Way more powerful than a simple looper block.
The Axe looper is pretty good, but does just a basic job. Dedicated hardware or software with a controller is more what you need to really do those one man band, lots of loops and layers. I think Ed Sheeran uses Ableton with a custom midi foot controller. Interesting setup with way more control than using just a simple pedal.